Re: [RFC PATCH 0/5] hugetlb: Change huge pmd sharing

[David Hildenbrand]

On 06.04.22 22:48, Mike Kravetz wrote:
> hugetlb fault scalability regressions have recently been reported [1].
> This is not the first such report, as regressions were also noted when
> commit c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
> synchronization") was added [2] in v5.7.  At that time, a proposal to
> address the regression was suggested [3] but went nowhere.
> 
> To illustrate the regression, I created a simple program that does the
> following in an infinite loop:
> - mmap a 4GB hugetlb file (size insures pmd sharing)
> - fault in all pages
> - unmap the hugetlb file
> 
> The hugetlb fault code was then instrumented to collect number of times
> the mutex was locked and wait time.  Samples are from 10 second
> intervals on a 4 CPU VM with 8GB memory.  Eight instances of the
> map/fault/unmap program are running.
> 
> v5.17
> -----
> [  708.763114] Wait_debug: faults sec  3622
> [  708.764010]             num faults  36220
> [  708.765016]             num waits   36220
> [  708.766054]             intvl wait time 54074 msecs
> [  708.767287]             max_wait_time   31000 usecs
> 
> 
> v5.17 + this series (similar to v5.6)
> -------------------------------------
> [  282.191391] Wait_debug: faults sec  1777939
> [  282.192571]             num faults  17779393
> [  282.193746]             num locks   5517
> [  282.194858]             intvl wait time 19907 msecs
> [  282.196226]             max_wait_time   43000 usecs
> 
> As can be seen, fault time suffers when there are other operations
> taking i_mmap_rwsem in write mode such as unmap.
> 
> This series proposes reverting c0d0381ade79 and 87bf91d39bb5 which
> depends on c0d0381ade79.  This moves acquisition of i_mmap_rwsem in the
> fault path back to huge_pmd_share where it is only taken when necessary.
> After, reverting these patches we still need to handle:
> fault and truncate races
> - Catch and properly backout faults beyond i_size
>   Backing out reservations is much easier after 846be08578ed to expand
>   restore_reserve_on_error functionality.
> unshare and fault/lookup races
> - Since the pointer returned from huge_pte_offset or huge_pte_alloc may
>   become invalid until we lock the page table, we must revalidate after
>   taking the lock.  Code paths must backout and possibly retry if
>   page table pointer changes.
> 
> The commit message in patch 5 suggests that it is not safe to use
> SPLIT_PMD_PTLOCKS for hugetlb mappings if sharing is possible.  If
> others confirm/agree then there will need to be additional work.
> 
> Please help with comments or suggestions.  I would like to come up with
> something that is performant and safe.

May I challenge the existence of huge PMD sharing? TBH I am not
convinced that the code complexity is worth the benefit.


Let me know if I get something wrong:

Let's assume a 4 TiB device and 2 MiB hugepage size. That's 2097152 huge
pages. Each such PMD entry consumes 8 bytes. That's 16 MiB.

Sure, with thousands of processes sharing that memory, the size of page
tables required would increase with each and every process. But TBH,
that's in no way different to other file systems where we're even
dealing with PTE tables.


Which results in me wondering if

a) We should simply use gigantic pages for such extreme use case. Allows
   for freeing up more memory via vmemmap either way.
b) We should instead look into reclaiming reconstruct-able page table.
   It's hard to imagine that each and every process accesses each and
   every part of the gigantic file all of the time.
c) We should instead establish a more generic page table sharing
   mechanism.


Consequently, I'd be much more in favor of ripping it out :/ but that's
just my personal opinion.

-- 
Thanks,

David / dhildenb